During the first course, patients receive gemcitabine IV over 30 minutes once weekly for 2 weeks followed by one week of rest. During subsequent courses, patients receive gemcitabine as above followed immediately by fluorouracil IV over 24 hours once weekly for 2 weeks. Patients also receive leucovorin calcium orally on days 1 and 8 and IV on days 2 and 9. Courses repeat every 3 weeks in the absence of disease progression or unacceptable toxicity.